Two-step roller finger followers (RFF) for controllably activating compression valves in a variable valve actuation train in an internal combustion engines are well known. An RFF extends between a hydraulic lash adjuster (HLA) and the stem of a valve. Engagement with a cam lobe of an engine camshaft causes the RFF to be pivoted about the HLA and thereby to depress the valve stem, opening the valve. A two-step RFF mechanism allows an engine valve to be operated by two different cam lobe profiles, one with the mechanism locked and the other with the mechanism unlocked. When the mechanism is unlocked, the RFF portion that is not directly in contact with the valve stem and the HLA, known in the art as the high-lift follower, typically is provided with a spring means, known in the art as a “lost-motion” spring, to keep that portion in contact with the cam. A typical lost-motion spring is disposed in compression between the high-lift follower and the remainder of the RFF, known in the art as the low-lift follower. Thus, when the high-lift follower engages the cam lobe, all lash is removed from the RFF and force begins to be exerted by the spring against the HLA. If the force of the lost motion spring is too small, the high-lift follower may not be able to stay in contact with the cam under all engine operating conditions. If the spring force is too large, the force of the lost motion spring may overcome the force of the internal spring in the HLA causing the HLA to leak down and become undesirably compressed and depleted of oil.
In some prior art two-step RFFs, a torsional lost-motion spring is disclosed. See, for example, U.S. Pat. No. 6,769,387. Experience has shown that a torsional lost-motion spring can have excessive variation in its free angle, resulting in excessive variation in the installed load, making it difficult to balance the force of the torsional lost motion spring from being too large a force and too small a force. Further, a torsion spring exerts substantial friction in use, resulting in undesirably large hysterisis, again affecting the installed load.
It is known to employ compression lost-motion springs. See, for example, US Patent Application Publication No. US 2003/02003/0209216. A disadvantage of compression springs as disclosed in this publication is that the springs are not guided. Because the opposing spring seats follow rotational rather than linear paths, the springs can flex as well as compress in use, resulting in unstable spring dynamics and uncontrolled spring rates.
Compression lost-motion springs have been found to have significantly less load variation and less friction than torsional springs. However, actually implementing compression springs for this purpose is difficult because of the non-linearity of the actuating path and the limited space available in a typical two-step RFF structure.
What is needed in the art is a two-step roller finger follower having an improved arrangement of a compression lost-motion spring wherein frictional losses are minimized, spring compression is substantially linear rather than rotational, and spring length and diameter are maximized.
It is a principal object of the present invention to reduce frictional hysterisis and improve RFF working life cycle.